Western blot analysis of PARP1, p53, and p21 proteins after 5 Gy IR. (A) Twenty-one of 40 B-precursor ALL tumors exhibited normal apoptotic responses after IR showing PARP1 cleavage and normal p53 and p21 protein accumulation. ALL 76 showed no p21 protein induction but did exhibit PARP1 cleavage. (B) Nineteen of 40 with B-precursor ALL tumors exhibited defective apoptotic responses after IR. Actin shows equal loading. (i) Fifteen with ALL tumors exhibited defective PARP1 cleavage after IR despite normal p53 and p21 protein accumulation. (ii) Three ALL tumors exhibited defective PARP1 cleavage and defective activation of p53 and p21 proteins after IR, suggesting a defect upstream of p53. (iii) One ALL tumor exhibited high basal levels of p53 protein with no further accumulation of p53 or p21 proteins after IR, suggesting a TP53 mutation. r indicates relapse sample; and *, delayed PARP1 cleavage.